An analog digital converter (ADC) is a circuit that converts an analog input signal into a digital output signal. In order to increase a sampling frequency of analog digital conversion, a time interleave ADC is proposed in which a plurality of ADCs (ADC channels) are provided and the plurality of ADCs convert an analog input signal into a digital output signal in order in a time division manner. The time interleave ADC is described in Japanese Patent Application Laid-Open No. 2005-348156 and Japanese Patent Application Laid-Open No. 2008-11189.
The time interleave ADC enables a high-speed operation. However, when characteristics of the ADCs are different or a relation among timings when the ADCs operate deviates, an S/N ratio sometimes falls. As a method of correcting an error among the ADC channels, foreground calibration and background calibration are proposed. The former requires time for correction other than normal operation time of the ADCs. On the other hand, since the latter performs correction during a normal operation of the ADCs, it corrects, in background, an error caused by aged deterioration, a temperature change, or the like. A background calibration circuit is described in S. M. Jamal, et al., “A 10 b 120 Msample/s Time-Interleaved Analog-to-Digital Converter With Digital Background Calibration”, JSSC 2002.
In the background calibration circuit, an adaptive filter is provided in at least one of a plurality of ADC channels that operate in a time interleave (time division) manner. A coefficient of the adaptive filter is calculated on the basis of an added-up output obtained by adding up outputs of the plurality of ADC channels. The coefficient of the adaptive filter is controlled to suppress a spurious signal component of the added-up output. Consequently, a spurious signal component (an error signal component or an image signal component), which is an error (a skew error) caused by deviation (skew) of sampling timing of the time interleave, is suppressed.
However, when a sinc function is used instead of a delta function as the coefficient of the adaptive filter, a desired phase shift control is possible as long as phase shift control is performed at accuracy of a tap of the adaptive filter. However, a characteristic of the adaptive filter has frequency dependency when the phase shift control is performed at accuracy smaller than the accuracy of the tap. Therefore, when an analog input signal has a high frequency, the desired phase shift control is difficult or impossible because of the frequency dependency.